In recent years, in view of global environment conservation, an improvement in fuel efficiency of automobiles has been strongly requested. Accordingly, by increasing the strength of materials used for forming automobile bodies, a decrease in thickness and a reduction in weight have been energetically carried out. However, the increase in strength of steel sheets may cause degradation in formability due to degradation in ductility and, hence, development of materials having a high strength and a high ductility at the same time has been desired.
Heretofore, as a material in response to the requirement as described above, composite microstructure steel sheets, such as transformation hardening type DP steel (Dual Phase Steel) composed of ferrite and martensite, and TRIP steel using the TRIP (Transformation Induced Plasticity) phenomenon of retained austenite, have been developed.
For example, in Japanese Unexamined Patent Application Publication Nos. 61-157625 and 10-130776, TRIP steel using strain-induced transformation of retained austenite has been disclosed. However, since this TRIP steel needs an addition of a large amount of Si, there has been a problem in that phosphatability and/or hot-dip galvannealed properties of steel sheet surfaces are degraded, and in addition, since an addition of a large amount of C is required to increase the strength, for example, there has also been a problem in that a nugget fracture at a spot-welded joint is liable to occur.
In addition, in Japanese Unexamined Patent Application Publication No. 11-279691, a hot-dip galvannealed steel sheet having superior formability has been disclosed which achieves a high ductility by securing retained γ by an addition of a large amount of Si. However, since Si causes degradation in Zn coatability, when Zn coating is performed on the steel as described above, a complicated step, such as pre-coating of Ni, application of a specific chemical, or reduction of an oxide layer on a steel surface to control the oxide layer thickness, must be performed.
In addition, in Japanese Unexamined. Patent Application Publication Nos. 05-247586 and 2000-345288, TRIP steel containing a reduced amount of Si has been disclosed. However, since this TRIP steel needs an addition of a large amount of C to ensure a high strength, a problem relating to welding has still remained and, in addition, since the yield stress is extremely increased at a tensile strength of 980 MPa or more, there has been a problem in that dimensional precision in sheet metal stamping are degraded.
Furthermore, in general, in the TRIP steel, since a large amount of retained austenite is present, at the interface between a martensite phase generated by the induced transformation in forming and a phase therearound, a large number of voids and dislocations are generated. Hence, it has been pointed out that at the place as described above, hydrogen is accumulated, and as a result, a delayed fracture is disadvantageously liable to occur.
On the other hand, although transformation hardening type DP steel composed of ferrite and martensite has been known as a steel sheet having a low yield stress and a superior ductility, to realize a high strength and a high ductility, an addition of a large amount of Si is required, and as a result, a problem of degradation in phosphatability and/or hot-dip galvannealed properties has occurred. Accordingly, in Japanese Unexamined Patent Application Publication Nos. 2005-220430 and 2005-008961, to ensure hot-dip galvannealed properties, a steel sheet has been disclosed in which the amount of Si is decreased and Al is added. However, it cannot be said that a sufficient ductility is realized.
As described above, by the conventional DP steel and TRIP steel, a high strength cold-rolled steel sheet simultaneously having a high strength and a high ductility, and also having superior phosphatability, Zn coatability and the like has not yet been realized. In addition, in the steel sheets described above, the variation in mechanical properties, in particular, the variation in tensile strength, is large when conditions of annealing performed in manufacturing are changed. Hence, there has been a problem in that manufacturing stability is not good enough.
Accordingly, it could be helpful to solve the above problems of the conventional techniques and provide a high strength steel sheet and a method for manufacturing the same, the high strength steel sheet having a tensile strength of 950 MPa or more and a high ductility; superior phosphatability and hot-dip galvannealed properties; and a small variation in mechanical properties with the change in conditions of annealing.